1. Field of the Invention
This invention relates to a method, device, and system for detecting a layer 2 loop.
2. Description of the Related Art
In a network that is constituted of using a layer 2 switch, “a layer 2 loop” that occurs due to false connection between cables or a failure of a device is conventionally recognized as one hazardous fault. When a layer 2 loop occurs, the whole area of broadcast domain (a range that broadcast packets reach) such as a subnet is flooded with a large quantity of broadcast packets. As a result, for example, a terminal performs processing that it receives a large quantity of broadcast packets once, checks information in an upper layer about each broadcast packet, and then discards them. Therefore, a high load is imposed on the terminal. A band is occupied by a large quantity of broadcast packets in a network, which may cause communication to put in an impossible condition. Along with wider Ethernet (trademark) or prevalence of virtual local area network (VLAN), a layer 2 loop that causes such difficulties becomes a more serious problem.
A method of using spanning-tree protocol (STP) prescribed in IEEE802.1D is considered to prevent occurrence of a layer 2 loop. However, it is necessary for a relay device such as a layer 2 switch to support STP and to keep a function of STP in operation all the time. Therefore, the method may be difficult to use based on environments of a network or an operation policy and is not a suitable solution. A method of introducing a relay device such as a layer 2 switch that has a function of preventing occurrence of a layer 2 loop is also considered to prevent occurrence of a layer 2 loop, however, because it is necessary to replace all of existing layer 2 switches, to realize introduction of the device is difficult and the method is also not a suitable solution.
Various methods to detect a layer 2 loop are proposed as disclosed in Japanese Patent Application Laid-Open No. 2001-197114, Japanese Patent Application Laid-Open No. 2006-33275, Japanese Patent Application Laid-Open No. 2004-364065, and Japanese Patent Application Laid-Open No. 2006-173785.
In the above conventional methods, a detecting device connected to a network to be detected or a detecting device included in a layer 2 switch to be detected analyzes packets received in the network to be detected or in the layer 2 switch to be detected to detect a layer 2 loop. A problem arises in that a layer 2 loop cannot be detected that occurs in a network different from the network to which the detecting device is connected (a network having a different network address, for example, a different subnet).
A method of connecting a detecting device to all of networks or a method that a network manager reaches a network in which a layer 2 loop is suspected to occur (a suspected network) and connects a detecting device to the suspected network is considered, however, the above problem cannot be solved in a suitable way.